Heating, ventilating and air conditioning (HVAC) systems typically divide a large building into zones, and the HVAC system is adapted to maintain each of the zones within predefined environmental parameters (e.g., temperature, humidity, outdoor-recirculated air ratio, etc.). Each zone has a set of air distribution ducts connected to an air handling unit (AHU) that provides a supply of conditioned air to the particular zone. The air handling unit typically includes elements for introducing outdoor air into the distribution system and for exhausting air from the system. Elements also are provided to heat, cool, filter and otherwise condition the air for the zone, as well for circulating the air within the zone's air distribution ducts at a desired flow rate.
Air flow from the air handling unit to each room within the zone is regulated by a separate variable air volume (VAV) terminal unit, also called a VAV box. The typical variable air volume terminal unit has a damper driven by an actuator to vary the flow of air from the zone's air distribution duct. In a common air distribution system a single duct extends from the air handling unit to supply air at a given temperature to each variable air volume terminal unit. In order to provide air at the proper temperature for a given room, the variable air volume terminal unit has heating and cooling elements element.
The operation of the variable air volume terminal unit is provided by a VAV controller that must provide both continuous and discrete control functions. Continuous control includes a temperature loop and a flow loop, while discrete control includes the sequencing of the heating device.
Another type of air distribution system utilizes a pair of ducts to furnish heated and cooled air to each variable air volume terminal unit. The VAV terminal unit mixes air from both ducts to supply air at the proper temperature to the room. Conventional control of this dual duct system is based on sensing the temperature of the air at different point as described in U.S. Pat. No. 5,558,274.